Combine the sugar, corn syrup, and water in a pan and bring to a rolling boil while stirring consistently. Clasp your candy thermometer to the side of the pan so it will help prevent the mixture from boiling over, and you will know when it hits 300 degrees.
When it reaches a boil bring the heat down so it maintains it's rolling boil but isn't ready to boil over. You can leave it there without stirring much until the candy thermometer reaches 300 degrees.
Keep your eye on it! Put food coloring in separate bowls. Pour equal amounts of hot mixture into bowls.
Use a spoon to fill silicone molds. Use a knife to scrape across top of molds if you want a really flat back to the Legos....not a requirement. Leave on counter overnight and pop out in morning....or can put in fridge if you want them done faster.